1. Field of the Invention
The present invention relates to a piezoelectric composition, a piezoelectric ceramic manufactured by using the piezoelectric composition, a transducer, and an ultrasonic motor.
2. Description of the Related Art
A piezoelectric ceramic used in an ultrasonic motor, an ultrasonic cleaner, a piezoelectric transformer, and the like, and a piezoelectric ceramic used in an actuator that utilizes a resonance displacement are required to have a large resonance displacement and an excellent piezoelectric property. Specially, when the piezoelectric ceramic is used in the ultrasonic motor or the like, a drive frequency becomes high, so that a low mechanical quality factor (Qm) of the piezoelectric ceramic leads to easy heat generation, which may cause depolarization, change in piezoelectric constant, or the like. Thus, the piezoelectric ceramic having a high mechanical quality factor (Qm) is required.
In the piezoelectric ceramic that satisfies such property, a PZT (PbZrO3—PbTiO3 solid solution) based and a PT (PbTiO3) based perovskite-type compound, and the like are used (see Japanese Patent No. 3785648). However, these compounds include Pb, which leads to environmental concerns. Therefore, a Pb-free compound has been variously studied.
For example, the piezoelectric ceramic using a BaTiO3 based compound or a KNN (KNbO3—NaNbO3 solid solution) based compound is proposed (see Japanese Laid-open Patent Publication No. 2003-055048). Moreover, a resonant actuator that exerts an excellent mechanical quality factor (Qm) by orienting crystals of a bismuth layer compound is also proposed (see Shinichiro KAWADA, Hirozumi OGAWA, Masahiko KIMURA, Kosuke SHIRATSUYU, and Yukio HIGUCHI, “Relationship between Vibration Direction and High-Power Characteristics of h001i-Textured SrBi2Nb2O9 Ceramics”, Japanese Journal of Applied Physics, Vol. 46, No. 10B, 2007, pp. 7079-7083 (hereinafter, “Kawada”) and Hirozumi Ogawa, Shinichiro Kawada, Masahiko Kimura, Kousuke Shiratsuyu, and Yukio Sakabe, “High-Power Piezoelectric Characteristics of Textured Bismuth Layer Structured Ferroelectric Ceramics”, IEEE transactions on ultrasonics, ferroelectrics, and frequency control, vol. 54, no. 12, December 2007, pp. 2500-2504 (hereinafter, “Ogawa”)).
However, the piezoelectric ceramic using a BaTiO3 based compound disclosed in Japanese Laid-open Patent Publication No. 2003-055048 has a low mechanical quality factor (Qm) compared with the piezoelectric ceramic using PZT-based and PT-based compounds. Therefore, this may not be suitable for the above use such as the ultrasonic motor. Moreover, the piezoelectric ceramic using a KNN-based compound disclosed in Japanese Laid-open Patent Publication No. 2003-055048 includes Nb in a raw material, so that a material cost is relatively high, and moreover, alkali metal such as K and Na is used, so that deviation of composition when firing is easy to occur and a sufficient moisture resistance may not be obtained.
Furthermore, a piezoelectric ceramic disclosed in International Publication No. WO2008/090758 may not sufficiently suppress self-heating when driving even if the mechanical quality factor (Qm) is made sufficiently high, whereby the piezoelectric property changes in some cases. Moreover, in the piezoelectric ceramics disclosed in Kawada and Ogawa, a high mechanical quality factor (Qm) is obtained by crystallographically orienting a bismuth layer component; however, many processes are needed for crystallographically orienting the bismuth layer component, therefore having a problem in cost and the like for industrially manufacturing it.